Germanium is used in a wide variety of applications, such as optoelectronics and quantum bit computing. For example, germanium is particularly useful in the manufacture of photodetectors, since conventional silicon photodetectors cannot detect the near-infrared light that is used for optical communications. Germanium films also advantageously offer good electrical carrier transport properties for integrated circuit applications and compatibility with existing silicon technologies. However, pure germanium wafers are relatively expensive, and production-worthy techniques for forming germanium thin films having the physical characteristics that are used in many applications are not available. Examples of such physical characteristics include etch pit density and surface roughness.
For example, one issue that often arises during the production of germanium films, and that can compromise the physical characteristics of germanium films, is the lattice strain that results from heteroepitaxial deposition. A “heteroepitaxial” deposited layer is an epitaxial or single crystal film that has a different composition than the single crystal substrate onto which it is deposited. A deposited epitaxial layer is said to be “strained” when it is constrained to have a lattice structure in at least two dimensions that is the same as that of the underlying single crystal substrate, but different from its inherent lattice constant. Lattice strain occurs because the atoms in the deposited film depart from the positions that they would normally occupy in the lattice structure of the free-standing, bulk material when the film epitaxially deposits so that its lattice structure matches that of the underlying single crystal substrate.
Heteroepitaxial deposition of a germanium-containing material, such as silicon germanium or germanium itself, onto a single crystal silicon substrate—such as a wafer or an epitaxial silicon layer—generally produces compressive lattice strain because the lattice constant of the deposited germanium-containing material is larger than that of the silicon substrate. The degree of strain is related to the thickness of the deposited layer and the degree of lattice mismatch between the deposited material and the underlying substrate. Additionally, greater amounts of germanium generally increase the amount of strain in the germanium-containing layer. Specifically, the higher the germanium content, the greater the lattice mismatch with the underlying silicon, up to pure germanium, which has a 4% greater lattice constant compared to silicon.
As the thickness of the germanium-containing layer increases above a certain thickness, called the critical thickness, the germanium-containing layer relaxes automatically to its inherent lattice constant. This relaxation requires the formation of misfit dislocations at the film/substrate interface. The critical thickness depends partially upon temperature: that is, the higher the temperature, the lower the critical thickness. The critical thickness also depends partially on the degree of mismatch due to germanium content: that is, the higher the germanium content, the lower the critical thickness. For example, a SiGe film containing about 10% germanium has a critical thickness of about 300 Å at about 700° C. for an equilibrium (stable) strained film and about 2,000 Å for a metastable, strained film on Si<100>.
If the strain is to be maintained, the thickness is kept below the critical thickness and a cap layer is applied to the strained heteroepitaxial layer. The cap layer helps to maintain the metastable strain of the germanium-containing layer during subsequent processing steps. For example, in certain applications, this is done to facilitate the formation of an emitter-base junction at the desired depth within the structure. In other applications, such as when forming a buffer for subsequent strained deposition, the germanium layer is relaxed. Strained semiconductor layers advantageously exhibit enhanced electrical carrier mobility and therefore greater integrated circuit speed, but relaxed semiconductor layers are advantageous for forming buffer layers that set the crystal lattice for overlying strained semiconductor layers.